Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, a noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbons (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reactions with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst has been used, such as a DeNOx catalyst. The LNT catalyst adsorbs the NOx contained in the exhaust gas when an air/fuel ratio is lean, and releases the adsorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich (hereinafter, it will be called a ‘regeneration of the LNT’).
Since materials which adsorb the nitrogen oxide in the LNT, however, are base materials, sulfur oxide (material obtained by oxidizing sulfur contained in fuel or engine oil) as well as the nitrogen oxide contained in the exhaust gas is adsorbed. Sulfur poisoning in the LNT deteriorates a purification efficiency of the LNT. Therefore, desulfurization of the LNT may be necessary.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.